The present invention relates generally to medical devices and more particularly to delivery systems for implantable medical devices, such as self-expanding stents.
Stents have become a common alternative for treating vascular conditions because stenting procedures are considerably less invasive than other alternatives. As an example, stenoses in the coronary arteries have traditionally been treated with bypass surgery. In general, bypass surgery involves splitting the chest bone to open the chest cavity and grafting a replacement vessel onto the heart to bypass the stenosed artery. However, coronary bypass surgery is a very invasive procedure that presents increased risk and requires a long recovery time for the patient. By contrast, stenting procedures are performed transluminally and do not require open surgery. Thus, recovery time is reduced and the risks of surgery are minimized.
Many different types of stents and stenting procedures are possible. In general, however, stents are typically designed as tubular support structures that may be inserted percutaneously and transluminally through a body passageway. Typically, stents are adapted to be compressed and expanded between a smaller and larger diameter. However, other types of stents are designed to have a fixed diameter and are not generally compressible. Although stents may be made from many types of materials, including non-metallic materials and natural tissues, common examples of metallic materials that may be used to make stents include stainless steel and nitinol. Other materials may also be used, such as cobalt-chrome alloys, amorphous metals, tantalum, platinum, gold, titanium, polymers and/or compatible tissues. Typically, stents are implanted within an artery or other passageway by positioning the stent within the lumen to be treated and then expanding the stent from a compressed diameter to an expanded diameter. The ability of the stent to expand from a compressed diameter makes it possible to thread the stent through narrow, tortuous passageways to the area to be treated while the stent is in a relatively small, compressed diameter.
Once the stent has been positioned and expanded at the area to be treated, the tubular support structure of the stent contacts and radially supports the inner wall of the passageway. The implanted stent may be used to mechanically prevent the passageway from closing in order to keep the passageway open to facilitate fluid flow through the passageway.
Stents may also be used in combination with other components to treat a number of medical conditions. For example, stent-graft assemblies are commonly used in the treatment of aneurysms. As those in the art well know, an aneurysm is an abnormal widening or ballooning of a portion of an artery. Generally, this condition is caused by a weakness in the blood vessel wall. High blood pressure and atherosclerotic disease may also contribute to the formation of aneurysms. Common types of aneurysms include aortic aneurysms, cerebral aneurysms, popliteal artery aneurysms, mesenteric artery aneurysms, and splenic artery aneurysms. However, it is also possible for aneurysms to form in blood vessels throughout the vasculature. If not treated, an aneurysm may eventually rupture, resulting in internal hemorrhaging. In many cases, the internal bleeding may be so massive that a patient can die within minutes of an aneurysm rupture. For example, in the case of aortic aneurysms, the survival rate after a rupture can be as low as 20%.
Traditionally, aneurysms have been treated with surgery. For example, in the case of an abdominal aortic aneurysm, the abdomen is surgically opened, and the widened section of the aorta is typically dissected longitudinally. A graft material, such as Dacron, is then inserted into the vessel and sutured at each end to the inner wall of the non-widened portions of the vessel. The dissected edges of the vessel may then be overlapped and sutured to enclose the graft material within the vessel. In smaller vessels where the aneurysm forms a balloon-like bulge with a narrow neck connecting the aneurysm to the vessel, the surgeon may put a clip on the blood vessel wall at the neck of the aneurysm between the aneurysm and the primary passageway of the vessel. The clip then prevents blood flowing through the vessel from entering the aneurysm.
An alternative to traditional surgery is endovascular treatment of the blood vessel with a stent-graft. This alternative involves implanting a stent-graft in the blood vessel across the aneurysm using conventional catheter-based placement techniques. The stent-graft treats the aneurysm by sealing the wall of the blood vessel with a generally impermeable graft material.
Thus, the aneurysm is sealed off and blood flow is kept within the primary passageway of the blood vessel. Increasingly, treatments using stent-grafts are becoming preferred since the procedure results in less trauma and faster recuperation.
Self-expanding stents are one common type of stent used in medical procedures. Self-expanding stents are increasingly being used by physicians because of their adaptability to a variety of different conditions and procedures. Self-expanding stents are usually made of shape memory materials or other elastic materials that act like a spring. Typical metals used in this type of stent include Nitinol and stainless steel. However, other materials may also be used.
Traditionally, self-expanding stents have been used in a number of peripheral arteries in the vascular system due to the elastic characteristic of these stents. One advantage of self-expanding stents for peripheral arteries is that traumas from external sources do not permanently deform the stent. As a result, the stent may temporarily deform during unusually harsh traumas and spring back to its expanded state once the trauma is relieved. However, self-expanding stents may be used in many other applications as well.
The above-described examples are only some of the applications in which stents are used by physicians. Many other applications for stents are known and/or may be developed in the future.
To facilitate stent implantation, self-expanding stents are normally installed on the end of an inner catheter in a low profile, compressed state. The distal end of the inner catheter is typically attached to a tapered catheter tip having a lumen. When placed on the inner catheter, the self-expanding stent is positioned just proximal to the catheter tip. The stent is typically inserted into an outer catheter at the end of the catheter, which restrains the stent in the compressed state. The outer catheter is also known as a sheath. The stent and catheter assembly is then guided to the portion of the vessel to be treated. Once the catheter assembly and stent are positioned adjacent the portion to be treated, the stent is released by pulling, or withdrawing, the outer catheter rearward. Normally, a stop member or other feature is provided on the inner catheter to prevent the stent from moving rearward with the outer catheter or sheath. After the stent is released from the retaining outer catheter, the stent springs radially outward to an expanded diameter until the stent contacts and presses against the vessel wall. The inner catheter and attached catheter tip are then withdrawn through the lumen of the stent, leaving the stent in place within the vessel.
As previously noted, in stent delivery systems, the distal end of the inner catheter is attached to a tapered catheter tip. The tapered catheter tip facilitates pushing the stent delivery system through tissue or blood vessels to the desired location. FIG. 1 shows a conventional catheter tip assembly 10 for use in a stent delivery system. As shown in FIG. 1, the catheter tip 12 has a tapered distal end 14, a tapered proximal end 16 and a longitudinally-running lumen 18. The proximal end 16 of the catheter tip 12 is attached to the distal end 20 of the inner catheter 22. The distal end 20 of the inner catheter 22 is sized such that it fits within the proximal portion of the lumen 18 of the catheter tip 12. An adhesive is used to secure the catheter tip 12 to the inner catheter 22. As shown, a stop member 24 may be located on the inner catheter 22, which extends radially outwardly from the inner catheter 22. During use, a self-expanding stent 26 in a compressed state may be loaded on the inner catheter 22 between the stop member 24 and the catheter tip 12.
The manner of attachment between the inner catheter 22 and the catheter tip 12 shown in FIG. 1 poses a risk that the catheter tip 12 will snag or catch the distal end of the self-expanding stent 26 as the inner catheter 22 and catheter tip 12 are withdrawn through the lumen of the expanded stent 26. This is because the juncture between the distal end 20 of the inner catheter 22 and the proximal end 16 of the catheter tip 12 forms an edge 28. This edge 28 may be prone to catching on the stent 26 because it faces the proximal direction. Thus, as the inner catheter 22 and catheter 12 are withdrawn in the proximal direction, the edge 28 may catch on the stent 26. If the catheter tip 12 catches on the stent 26, it may dislodge the stent 26 from the desired location in the patient's body. The catheter tip 12 may also separate from the inner catheter 22 if it catches on the edge of the stent 26, leaving the catheter tip 12 lodged in the vessel where it may block the vessel or cause other complications.
The manner of attachment between the inner catheter 22 and the catheter tip 12 shown in FIG. 1 may also result in the formation of an undesirable gap 34 between a stent 26 loaded on the inner catheter 22 and the stop member 24 on the inner catheter 22. Typically, in conventional systems, the distal end 20 of the inner catheter 22 is threaded through the lumen of a compressed stent 26 and the outer catheter 30 is placed over the compressed stent 26 to prevent it from expanding. After the stent 26 is loaded, the catheter tip 12 is attached to the inner catheter 22, usually with an adhesive. In order to accommodate the process of bonding the inner catheter 22 to the catheter tip 12, a gap 34 may be formed between the stop member 24 and the proximal end 32 of the stent 26. For example, the gap 34 may be between 3 mm and 1 cm in length, as measured in the axial direction. When the outer catheter 30 is withdrawn, the stent 26 initially moves proximally with the retention sheath through the gap 34 until the proximal end 32 of the stent 26 contacts the distal end of the stop member 24. Once the proximal end 32 of the stent 26 contacts the distal end of the stop member 24, the stop member 24 prevents the stent 26 from continuing to move proximally, thereby resulting in relative movement between the outer catheter 30 and the stent 26. However, because the stent 26 initially moves proximally with the outer catheter 30 through the gap 34, a slight delay in deployment may occur. This delay in deployment may cause inaccuracy in placement of the stent.